Heretofore, so-called resin laminates have been adopted as interior components for automobiles, components for building, and packaging components for physical distribution. A resin laminate includes a front-side member made of resin and a back-side member made of resin. The back-side member is provided with a cup-shaped portion the bottom of which butts against an inner surface of the front-side member. In cases of interior components for automobiles and components for building, to which outward appearances are of importance, particularly, a nonwoven fabric is laminated on a front side of the front-side member. Heretofore, various methods have been adopted as a method for manufacturing such a resin laminate. First, there has been adopted a technique of manufacturing a resin laminate from a molten resin through integral extrusion blow molding. The resin laminate manufactured by this method has a cup-shaped portion for connecting between a front-side member and a back-side member. As compared with a resin laminate having a double-wall hollow structure that a hollow portion is formed merely therein, therefore, this resin laminate can ensure stiffness, particularly, compressive stiffness relative to a load to be applied to a surface member in a vertical direction.
Second, there has been adopted a technique disclosed in Patent Document 1. According to this technique, two molten sheets, which are extruded independently of each other, are caused to pass between a pair of rolls one of which is a roll having a large number of protrusions arranged in a staggered pattern on a surface thereof, with a predetermined pressing force applied to each sheet. Thus, a plurality of cup-shaped portions are formed on one of the sheets. Moreover, the two sheets are welded to each other in a state that the bottom of each cup-shaped portion butts against an inner surface of the other sheet. Further, a different sheet is welded to a surface of the one sheet where an opening of each cup-shaped portion is formed. Herein, the resultant resin laminate has a three-layer structure that the different sheets are welded to the front and back of the sheet having the cup-shaped portion formed thereon, respectively.
Third, there has been adopted a technique disclosed in Patent Document 2, which is different from the second method. According to this technique, two molten sheets, which are extruded independently of each other, are caused to pass between a pair of rolls each of which is a roll having a large number of protrusions arranged in a staggered pattern on a surface thereof, with a predetermined pressing force applied to each sheet. Thus, a plurality of cup-shaped portions are formed on each of the sheets. Moreover, the two sheets are welded to each other in a state that the bottoms of the corresponding cup-shaped portions on the respective sheets are welded to each other. Further, a different sheet is welded to a side of each sheet opposed to the welded side. Herein, the resultant resin laminate has a four-layer structure that the different sheets are welded to the sides of the respective sheets where an opening of each cup-shaped portion is formed. The foregoing techniques of manufacturing a resin laminate by extrusion of a molten sheet each have the following technical problems.
That is, it is difficult to obtain a resin laminate having satisfactory strength with no anisotropy while ensuring manufacture efficiency. More specifically, the first to third methods have a problem unique to extrusion molding, in common. That is, in the extrusion molding, the sheet has an open end in a direction of extrusion, which indispensably requires facing treatment such as heat sealing treatment. As the result, an extra step is required for manufacturing the resin laminate, which leads to reduction in total manufacture efficiency. Further, the welding of the sheet on which the cup-shaped portions are arranged in a staggered pattern to the other sheet or the welding of the sheet on which the cup-shaped portions are arranged in a staggered pattern to the sheet on which the cup-shaped portions are also arranged in a staggered pattern is merely effected by the pressing force to be applied at the time when both the sheets pass between the pair of rollers while being fed by the rollers. As the result, a time required for the welding is insufficient, which makes it difficult to obtain the resin laminate having satisfactory strength and degrades the quality of the resin laminate. However, blow molding rather than the continuous extrusion molding allows prevention of the reduction in manufacture efficiency due to the facing treatment and the reduction in strength due to the insufficient welding. Patent Document 3 discloses a method using the blow molding. According to the technique disclosed in Patent Document 3, a tubular molten parison is used. A back wall has a cup-shaped portion the bottom of which butts against and is welded to an inner surface of a front wall, and a mounting material is laminated on an outer surface of the front wall. However, the blow molding for molding a tubular parison by application of a blowing pressure causes the following technical problem which is different from the foregoing problem. That is, it is difficult to satisfactorily reduce the weight and wall thickness of a resin laminate since the resin laminate is manufactured by molding a tubular parison having a uniform wall thickness in a circumferential direction by application of a blowing pressure.
More specifically, a tubular parison has a substantially uniform thickness in a circumferential direction because it is normally extruded from an annular slit between die cores. In blow molding, on the other hand, in a case of clamping a pair of split mold blocks, a blowing pressure is applied from a hermetic space formed between the mold blocks. Therefore, a force to be applied to press the parison against the mold block is uniform throughout the parison. However, the parison to be pressed against one of the mold blocks having the cup-shaped portion formed thereon is elongated because of a relation with a blow ratio according to a depth of a cup-shaped portion and a diameter of an opening to have a locally thin wall portion. On the other hand, the parison to be pressed against the other mold block having no cup-shaped portion formed thereon has no thin wall portion. In view of this point, the thickness of the tubular parison must be set based on the thin wall portion on the one mold block side, so that the sheet on the other mold block side has an unnecessary thickness. As described above, in the case of using a tubular parison having a substantially uniform wall thickness in a circumferential direction, a wall surface on which a plurality of cup-shaped portions are formed after blow molding and a wall surface on which no cup-shaped portions are formed are irreversibly difficult in thickness from each other. Because of this reason, it is impossible to achieve satisfactory reduction in weight and wall thickness of a resin laminate. With regard to this point, Patent Document 4 discloses a method for manufacturing a thermoplastic resin plate. Herein, the plate is manufactured from two molten sheets based on a tubular parison. The plate has a hollow portion formed therein, and two opposed sides each provided with a plurality of cup-shaped portions. The respective cup-shaped portions are formed such that the bottoms thereof are arranged back to back. However, in cases of resin laminates used as interior components for automobiles and components for building, to which outward appearances of front surfaces are of importance, a decorative material such as a nonwoven fabric needs to be laminated on the one sheet serving as the front surface. For this reason, it is preferable to prevent openings of a large number of cup-shaped portions from being formed on a surface of the one sheet.
Patent Document 1: Japanese Patent No. 4327275
Patent Document 2: Japanese Patent No. 4192138
Patent Document 3: JP 11-105113 A
Patent Document 4: JP 07-171877 A